A hormone that regulates appetite may also influence memory formation, researchers say. Recent findings, and the worldwide trend toward larger waistlines, have convinced them that we need to take a closer look at how obesity affects learning.

In the past decade, a hormone known as leptin has received enormous attention for its role in regulating appetite and metabolism. Fat tissues produce significant quantities of this natural compound, which in turn reduces a person's desire to eat.

At first, researchers were surprised to discover that obese individuals often over-express this hormone, as this would be expected to reduce their appetite. But many now believe that these people are simply desensitized to leptin.

At the same time, studies have begun to link memory deficits with metabolic diseases such as obesity and diabetes. Jenni Harvey of the University of Dundee, UK, says patients with diabetes can experience "anything from short-term memory loss to Alzheimer's-like symptoms". And she says leptin is the key.

An increasing number of reports show that abnormal levels of leptin can significantly alter brain cell function, Harvey told an audience at the annual meeting of the Biochemical Society in Glasgow, UK, this week.

Consuming knowledge

Biologist Matthew Wayner of the University of Texas at San Antonio agrees. His experiments on rats show that too much leptin can hamper the ability of brain cells to respond to a signal.

The hormone seems to affect a process known as long-term potentiation. In this process, nerve cells become more sensitive to the same stimulus with repeated exposure. Scientists think this provides a cellular basis for memory and learning.

Wayner's team injected leptin into the hippocampus, a brain region responsible for storing information, in rats. The researchers found that this increased the long-term potentiation of neurons1, which would be expected to improve learning.

From studies of individual neurons isolated in a dish, Wayner found that the amount of leptin produced by normal food intake in rats boosts long-term potentiation by 3 times. But increasing the dose 100-fold abolishes this effect.

Weighty concerns

So could abnormally high leptin levels in children with weight problems be hindering their ability to perform in school? Harvey says that given the increasing incidence of obesity in the young, researchers should investigate the potential link.

She and others add that understanding the role of leptin secretion in metabolic disorders will help explain why these illnesses increase the risk of impaired cognitive function. "It may tell us about what goes wrong in these diseases, which could serve as targets for treatments."